use same way to output version and expose build info to prometheus

[cpanato]

Summary

• export fulcio build/version information
• use the same way to expose version information as we use in rekor and other projects

with this, we can add a dashboard to see what version we have deployed and also can be used to compare between environments which I saw a issue around but was not able to find it

also fixes partially: https://github.com/sigstore/public-good-instance/issues/714

# HELP rekor_build_info A metric with a constant '1' value labeled by version, revision, branch, and goversion from which rekor was built.
# TYPE fulcio_build_info gauge
fulcio_build_info{build_date="2022-10-07T16:22:46",goversion="go1.19.2",revision="f2f24ac39392e366949a50972c8851bde2d39b7a",version="v0.6.0-9-gf2f24ac"} 1

[codecov-commenter]

Codecov Report

Merging #815 (f2f24ac) into main (7dc98ad) will increase coverage by 0.21%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #815      +/-   ##
==========================================
+ Coverage   55.56%   55.78%   +0.21%     
==========================================
  Files          37       38       +1     
  Lines        2354     2334      -20     
==========================================
- Hits         1308     1302       -6     
+ Misses        950      937      -13     
+ Partials       96       95       -1     

Impacted Files	Coverage Δ
cmd/app/root.go	37.50% <ø> (-6.95%)	⬇️
pkg/server/metrics.go	0.00% <0.00%> (ø)
cmd/app/version.go	100.00% <100.00%> (+68.18%)	⬆️

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[haydentherapper]

Thanks!